This invention relates to a noise reduction apparatus for reducing the noise that is caused by the propagation of periodical mechanical vibrations, by generating the sound waves which are calculated from the frequencies of the mechanical vibrations so as to be in antiphase with the noise waves and hence to actively cancel out the noise, and more particularly to a noise reduction apparatus suited to avoid the increase of the noise when the noise reduction effect cannot be obtained.
When a mechanical vibration source is periodically vibrating in the neighborhood, noise is generated by the propagation of the mechanical vibration. Automobiles and vessels have engines which act as the periodical mechanical vibration sources, and the vibrations of the wings of airplanes also act as the periodical mechanical vibration sources. This noise depends on the frequencies of the mechanical vibrations, and thus the frequencies of the noise can be known. However, since the ceilings, floors, walls, windows or others of the cabins of cars, vessels and so on can be resonated by the propagated mechanical vibrations, it is often not known where the actual noise comes from. Thus, a noise reduction apparatus has been developed for cancelling out the noise. According to this apparatus, secondary sound waves which are in antiphase with the noise are determined from the frequencies of the mechanical vibrations and the spatial acoustic transfer function of the cabin or the like, and the secondary sound waves are emanated within the cabin so as to cancel out the noise.
FIG. 1 shows a schematic block diagram of the noise reduction apparatus. This noise reduction apparatus includes microphones 4 for detecting the sound pressures at a plurality of locations within a noise space such as a cabin, a plurality of loudspeakers 5 for emanating secondary sound waves within the noise space, and a controller 3 having a microprocessor 2 as computation means. When mechanical vibrations are propagated from an engine 1 to a cabin or the like, noise is generated within the cabin by the mechanical vibrations. The microprocessor 2 considers the spatial acoustic transfer function of the noise space and calculates the secondary sound waves for actively cancelling out the noise from the mechanical vibration frequencies. The secondary sound waves are emanated from the loudspeakers 5 within the cabin, thereby reducing the noise within the cabin. At this time, the microprocessor 2 utilizes, for example, the least mean square algorithm (hereinafter, called the LMS algorithm) as a kind of the saddle point method, and calculates the secondary sound waves which are to be emanated from the loudspeakers 5 in order that the reverberant sound within the cabin which is detected by the microphones 4 can be minimized to be converged, not diverged.
There is a published in UK Patent Application Gazette No. 2149614A and a Japanese Patent Publication No. 1-501344, associated with the above-mentioned prior art.
In the above-mentioned noise reduction apparatus, while the secondary sound waves are being emanated for cancelling out the noise, the power supply is maintained to be in the on-state so that the noise reduction control function is always active. However, it has been found that during the operation of the noise reduction apparatus, the spatial acoustic transfer function between the microphones and the loudspeakers changes drastically. If, for example, the room temperature or the temperature outside the room or cabin is suddenly changed, the noise reducing effect disappears due to, for example, the change of the characteristics of the microphones with the change of temperature or the change of the air density within the cabin, with the result that the noise is reversely increased by the secondary sound emanation. It is troublesome for a crew-member or others to switch off the noise reduction apparatus each time the noise is increased. In addition, it is too late that after the noise is increased, the noise reduction apparatus is switched off by a crew-member or others, or the specially provided noise reduction apparatus becomes useless in that case. In order to widely spread the noise reduction apparatus, it is necessary to solve this problem.
Accordingly, it is an object of the invention to provide a noise reduction apparatus capable of preventing the noise from increasing by the secondary sound emanation even if the spatial acoustic transfer function of the noise space changes drastically.
The above object of the invention can be achieved by providing a noise reduction apparatus having noise detection means for detecting the noise generated by the propagation of a mechanical vibration, a digital computer for calculating from the frequency of the mechanical vibration the secondary sound which is opposite in phase to the noise, a D/A converter for converting the digital signal of the secondary sound calculated by the digital computer into an analog signal, a power amplifier for amplifying the analog signal produced from the D/A converter, and secondary sound generating means for generating secondary sound according to the analog signal amplified by the power amplifier and thereby cancelling out the noise to reduce the noise, wherein divergence detection means is further provided for monitoring the value of the digital signal and, when the value of the digital signal is shifted out of a normal value range, automatically interrupting the secondary sound emanation from the secondary sound generating means or for monitoring the value of the analog signal produced from the D/A converter and, when the value of the analog signal is shifted out of the normal value range, automatically interrupting the secondary sound emanation from the secondary sound generating means or for monitoring the analog signal produced from the power amplifier and, when the value of the analog signal is shifted out of the normal value range, automatically interrupting the secondary sound emanation from the secondary sound generating means.
When the noise is predicted to be increased by the emanation of the secondary sound, the divergence detection means automatically stops the generation of the secondary sound. Therefore, the noise can be satisfactorily reduced, and the persons within the cabin would not be annoyed by the increase of the noise.